[ RadSafe ] [Radsafe] equilibrium factors for thoron?

Wed Sep 9 17:44:08 CDT 2009

I believe the equilibrium ratio between Radon -222 and its decay products  
outdoors, is closer to 0.7 not 0. 2 as stated by Nick.. Measurements about 1 
 meter above the soil surface show less less plate-out on the surface and 
as  you go higher the equilibrium gets even greater.

Andy George.

In a message dated 9/9/2009 12:01:40 P.M. Eastern Daylight Time,  
eic at shaw.ca writes:

Hi  Nick,

I think the equilibrium factor for thoron can vary widely,  depending on 
the 
situation.

First, lets look at a room model. To  get an equilibrium factor of 0.4 for 
Rn-222, you need a mechanical  (ventilation and plateout) radon progeny 
removal rate  equivalent to  1.7 air changes per hour. See  
http://members.shaw.ca/eic/Tools/RoomRadon.htm

With a similar  removal rate, the equilibrium factor for Rn-220 is about 
0.04. (It can be  approximated by the ratio of the decay constant of 
Pb-212, 
0.063/h, and  the mechanical Pb-212 removal rate i.e. 1.7/h, in this 
example.) In a  situation with more air changes than in a house, i.e. most 
industrial  facilities, the equilibrium factor would be even lower.

However, in a  different physical situation, you can get equilibrium 
factors 
greater than  1. Suppose you are 10 minutes downwind of a Rn-220 source. 
Most 
of the  Rn-220 will have decayed and you are left with mostly Pb-212 and 
your  
equilibrium factor is 1.5. (The original Rn-220 concentration has been  
reduced by 10 half lives (i.e. a factor of 1/1024), while the activity of  
the resulting Pb-212, compared to the original Rn-220 activity, is given  
by 
the ratios of the half lives of Rn-220 and Pb-212 (1 minute / 11 hours  = 
1/660) and the equilibrium factor is 1024/660 = 1.5)

On the other  hand, if you are 10 minutes downwind of a Rn-222 source, the 
equilibrium  factor is 0.2 see 
http://members.shaw.ca/eic/Tools/Mine_Tunnel.htm . So,  in these 2 
different 
scenarios, the Rn-222 progeny equilibrium varied  between 0.4 and 0.2, 
while 
the Rn-220 progeny varied between 0.04 and 1.5,  which is a much greater 
range.

Bq/m3 EEC, is a (very confusing)  unit of potential alpha energy 
concentration and has very little to do  with the unit of Bq/m3, which is a 
unit of activity concentration. So, if  a regulator sets a limit of X Bq/m3 
EEC, he is limiting the potential  alpha energy concentration in air and, 
therefore, the dose to anyone  breathing this air. This is a reasonable 
thing 
to do and it does not  involve any assumption about equilibrium factors.

BTW, I thought  Australia was using µJ/m3 as the unit for the potential 
alpha 
energy  concentration in air, rather than Bq/m3 EEC. Can you confirm this? 
In  
North America, we use WL and I thought Bq/m3 EEC was used mainly by the  
former eastern block countries. (We offer an on-line radiation data  
management system for U mines and we should get the units used by  
different 
countries right!)

Hope this helps.

Kai  Kaletsch
Environmental Instruments Canada Inc.
#202 135 Robin  Cr.
Saskatoon, Saskatchewan
Canada S7L  6M3
http://www.eic.nu/
http://www.webrad.biz/

-----  Original Message ----- 
From: "Nick Tsurikov"  <nick.tsurikov at gmail.com>
To: <radsafe at radlab.nl>
Sent:  Wednesday, September 09, 2009 1:20 AM
Subject: [ RadSafe ] [Radsafe]  equilibrium factors for thoron?

> Dear  colleagues,
>
> I am getting a bit lost...  We all know the  equilibrium factors for
> Rn-222:
> 0.2 for outdoors and 0.4 for  indoors, which work perfectly fine - at 
least
> in a first approximation  for assessing the doses.
> However, the same equilibrium factor for  thoron (Rn-220) is quite a
> mystery
> (maybe just for me...). As  there is more equipment now that provides a
> direct measurement of  radon and thoron concentrations (which is quite
> convenient) I am  having trouble finding any references to convert the
> Bq/m3
> to  dose.  Some rather interesting numbers were quoted to me last week  -
> for
> example that the thoron equilibrium factor may be as low  as 0.01, on the
> other hand other people telling me that it may be up  to 0.6-0.7.
> It will be very much appreciated if you could point me out  in the proper
> direction - some regulatory documents, studies, whatever  - that I can use
> in
> trying to compare the levels I am  measuring with the limits that are in
> some
> places set up for  'equilibrium equivalent concentrations' - EEC.  The
> regulators  who wrote the document put there the values of 60 Bq for EEC 
of
> Rn-222  and 10 Bq/m3 for EEC of Rn-220, but are not sure themselves what
>  equilibrium factors were used; and your help will be very much
>  appreciated.
>
> Kind regards
> Nick Tsurikov
>  Western Australia
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